Decommissioning and Dismantling of the Moata Reactor-

A Radiation Protection Perspective

Prashant Maharaj

Australian Nuclear Science & Technology Organisation

A History of Moata

• Graphite moderator / reflector

• Cooled by light water

• Shielding - high density and low density concrete

• Constructed at ANSTO in 1961• ARGONAUT type reactor • Built as a 10 kW reactor, but modified

in 1972 to 100 kW

The Moata Reactor

From 1961 to 1995:– Neutron

radiography– Soil analysis– Cancer

treatment research

– Quality Control for HIFAR

Shutdown & Decommissioning

Two phases of dismantling:

– Preliminary Dismantling – the removal of the internal components of the reactor, including the steel core structures, graphite moderator and beam line facilities

– Structural (Biological Shield) Dismantling – the cutting and removal of the concrete shielding and the floor area below the shielding.

1995 1996 1997 1998 2009

Shutdown after 4,519 start-ups

Fuel unloaded

Decomm

issioning Project Team

established

Primary coolant (light water)

drained

Reactor Control System rem

oved

Planning comm

enced for

dismantling

Preliminary Dism

antling began

Preliminary Dism

antling complete

Containment tent installed

2010

Biological Shield Dismantling began

Biological Shield Dismantling Com

plete

Structural (Biological Shield) Dismantling

Structural (Biological Shield) Dismantling

• Reactor housed in a building used for other operational task

• A containment tent with HEPA filtered extraction system built around the reactor

Getting There…The steps involved in planning, completing and evaluating the

success of the Biological shield Dismantling:– Radiation Protection Plan

• Characterisation• Radiation Management & Monitoring• Dose Estimation & Constraints• Personal Protective Equipment• Final Status Surveys

– Outcomes & Lessons Learnt

Characterisation – Survey

• A radiological survey was carried out to determine the dose rates within the empty cavity areas.

• The data from this exercise was used to prepare a dose estimate for the Biological shield dismantling.

• This helped us estimate the maximum individual and collective dose for this part of the project.

Measured dose rates ranged from 7-4000 Sv/hr

Dose Estimation and Constraints

TASK SET 2

Main Bioshield (Heavy Density Concrete) Active Zone

Estimated time to perform task

Est. Exp Time (hours) for most exposed individual

Estimated Doserate (µSv/hr) for Worker 1,2,3 or 4

Estimated Dose Received (µSv) Worker 1-Cardinal

Estimated Dose Received (µSv) Worker 2-Cardinal

Estimated Dose Received (µSv) for Worker 3-Cardinal

Estimated Dose Received (µSv) for Worker 4-Cardinal

Estimated Doserate (µSv/hr) for HPS 1

Estimated Dose Received (µSv) HPS 1

Estimated Collective dose man-Sv

Machine demolition-Remote operation 24 12 70 840 840HPS Monitoring surveys 8 8 70 560Bobcat waste removal 8 4 130 520 520Material removal with 5tonne excavator 16 8 70 560Manual filling of waste containers 1 0.5 300 150Total For Cutting and Removal Of Main Bioshield (Active concrete) 57 840 840 520 1230 560

Summary Dose = 840 840 520 1230 560 3990

Dose Estimates:• Collective – 10,400 man-μSv• Av. Individual – 1156μSv (assuming 9 workers)• Max. Individual – 1444μSv

Dose Constraints:• Individual – 1500μSv• Daily – 50μSv

Radiation Management & Monitoring

Controlling exposure using distance

Radiation Management & MonitoringControlling exposure using shielding

Steel Plates for shielding

Radiation Management & MonitoringControlling exposure using shielding

Shielded containers for activated waste:

– Specially designed 6mm steel boxes for active concrete

Radiation Management & Monitoring

Dose Management:– Radiological

surveys

Dose Monitoring:– EPD System– TLDs– Whole Body

Monitoring

Radiation Management & Monitoring• External gamma monitors

and air samplers were placed close to work areas.

• Barrier and exit monitoring equipment were used to ensure that no contamination was taken out of the classified areas.

Personal Protective Equipment• Since there was a high potential of

generating dust while cutting and demolishing the active concrete it was decided to start from the highest level of PPE which included ;

Respiratory protection with particulate cartridge)

Tyveks

Overshoes and gloves

• This was than downgraded as we gained operational understanding of the project risks.

Final Status Survey• Dose rate surveys were carried out on

marked survey points (grid survey)• Acceptance criteria of 0.5uSv/hr was

met to ensure a brown-field condition was met

• Samples were taken in and around the foundations of the reactor and analysed for activity.

• The following guides were also used to determine the exemption and clearance criteria for the site

IAEA No. RS-G.1.7

ARPANSA Reg 1999, Schedule 2, Part 2

Outcomes & Lessons Learnt– Estimated doses vs. actual doses:

Dose ANSTO Estimated

ARPANSA Constraint

ANSTO Constraint Actual

Collective 10,400 person- μSv

13,000 person- μSv

10,400 person- μSv

1,679 person- μSv

Max. Individual 1444 μSv 1750 μSv 1500 μSv 252 μSv

Daily - - 50μSv 46μSv (max)

Outcomes & Lessons Learnt• From a radiation protection perspective the Biological Shield

Dismantling was completed safely and without incident.

– Dose estimates were higher than those recorded by staff. This was attributed to being overly conservative and some of the contributing factors were;

• Self shielding by the dismantled concrete • Time taken to dismantle and remove the active concrete was

less than anticipated. – No surface or airborne contamination was detected

throughout the project– There were no personal contamination events